STATCOM (Static synchronous Compensator) converters based on chain-link topology are well known and provide reactive power to high-voltage electricity transmission systems and are used e.g. for regulating voltage and stabilizing the system. The STATCOM converter comprises a number of converter cells, each converter cell comprising valves arranged in an H-bridge connection with a DC capacitor.
Occasionally, the STATCOM converter needs to be shut down, for example due to scheduled maintenance work or because of a required repair or exchange of components. When shutting down the STATCOM converter, DC capacitors thereof have to be discharged. This is accomplished by means of leakage current of the DC capacitors and by means of circuitry provided in the converter cells.
However, the discharge time is long, and the shutting down of the STATCOM converter is therefore a time consuming procedure. In case the STATCOM converter needs repair in order to be put back into operation, the availability of the converter is limited during a rather long time before reaching voltage levels that are safe for allowing personel to approach the STATCOM converter.